The present invention relates to a polycarbon sulfide usable as an active material for a battery, a process for preparing the same and a nonaqueous electrolytic battery comprising the same.
With the rapid expansion of portable electronic devices on the market, batteries to be used as power supplies thereof are more severely demanded to have higher performance. Besides, the development of batteries, which are more friendly to the global environment, is required. Under such circumstances, sulfur and its derivatives, which have a higher capacity, are more cost effective and give less loads on the environment, are expected to be promising as active materials for the positive electrodes of nonaqueous electrolytic batteries (primary batteries or secondary batteries).
If it is possible to utilize the two electron reaction of sulfur for a battery, sulfur will be an active material having an energy density as high as 1675 mAH/g theoretically. However, since sulfur is highly insulating and poor in reversibility, alkali metal-sulfur batteries have low capacity factors in practical use in the present state of art. In addition, the alkali metal-sulfur batteries can be used only under high temperature atmospheres, which leads to the corrosion of battery cases and the like due to the high activity of sulfur or its derivatives. Thus, it is hard to apply such batteries to miniature batteries for use in appliances.
In the meantime, inorganic sulfur compounds soluble in organic solvents, such as the sulfides of alkali metals are also used as active materials for the positive electrodes of batteries (see JP-A-57-145272, etc.). Porous carbon electrodes are used as the positive electrodes of batteries using the above inorganic sulfur compounds. These batteries are able to discharge at a larger quantity of current than the conventional sulfur batteries, but they are used mainly as primary batteries because carbon composing the electrodes is subject to deterioration during the discharge.
Further, organic sulfur compounds comprising carbon, sulfur, etc. as main constitutive elements have been investigated in possibility of usage as active materials for positive electrodes. For example, WO85/01293 (corresponding to JP-A-60-502213) proposes an organic sulfur compound represented by the formula: (RaCSb)b wherein R is a hydrogen, an alkali metal or a transition element. The present inventors have examined the syntheses of the organic sulfur compounds disclosed in the same publication and found that they have the following problems.
That is, it is impossible to perfectly substitute halogen atoms or hydrogen atoms with sulfur atoms in the synthesis by adding sulfur to a polymer such as a halogenated polyethylene (e.g. polytetrafluoroethylene, polytrifuluorochloroethylene or the like) or polyacetylene. Therefore, the resultant organic sulfur compounds tend to have many halogen atoms, hydrogen atoms or the like remaining in the molecules. In addition, it is impossible to control the amount of sulfur to be added, and therefore, it is very difficult to produce a compound having an uniform structure. This problem is manifest from the facts that the organic sulfur compounds synthesized in Examples 1 to 3 and 7 in WO85/01293 contain many elements other than carbon and sulfur.
Example 6 of WO85/01293 describes a product of the composition formula: CS0.98H0.009, which is composed of substantially two kinds of elements, i.e., carbon and sulfur. The present inventors have carefully examined the product and found that the synthesis described in Example 6 affords a mixture of an organic sulfur compound having a low sulfur content and a polysulfide compound. Since the polysulfide compound can not be removed by washing with water, it seems that the composition formula of the product described in Example 6 expresses an average composition of the above mixture, as a matter of fact. In addition, since the polymer containing no unsaturated bond is used as the starting material, the carbon backbone of the synthesized organic sulfur compound having a low sulfur content is essentially a carbon chain with saturated bonds, and further, since the number of the disulfide linkages (Cxe2x80x94Sxe2x80x94Sxe2x80x94C) with the carbon backbone present in the molecule is small, such an organic sulfur compound has difficulties in reversible charge and discharge, and also has poor discharge capacity. In other words, it is impossible that the synthesis described in WO85/01293 affords an organic sulfur compound consisting of two kinds of elements, i.e., carbon and sulfur and having a high capacity and a high sulfur content.
As another type of compound, an organic sulfur compound represented by the formula: (CSw)p wherein w is a number of 2.5 to about 50, and p is a number of 2 or more attracts attentions because the compound has an energy density as high as 1000 to 1600 mAh/g. Skotheim et al. use this compound as an active material for the positive electrode of a nonaqueous electrolytic battery and propose a secondary battery capable of having a high capacity at room temperature (see U.S. Pat. No. 5,441,831 (corresponding to JP-A-7-29599), WO96/41388 (corresponding to JP-A-11-506799), WO96/41387 (corresponding to JP-A-11-514128), etc.). This organic sulfur compound can be produced by any of the following methods: a method comprising reacting sodium sulfide with sulfur, followed by a reaction with an organic chloride compound; and a method comprising reacting acetylene with sulfur in a solution of metal sodium in ammonia; other is by reacting carbon disulfide with dimethylsulfone in the presence of metal sodium as a catalyst. This organic sulfur compound is characterized in that the molecular structure has a backbone having a conjugated structure formed mainly by carbon atoms and a structure represented by the formula xe2x80x94Smxe2x80x94 (mxe2x89xa73) (hereinafter referred to as polysulfide segment) bonded to the backbone.
However, it is impossible to design the molecule of the above organic sulfur compound of the formula: (CSw)p in the course of the synthesis, and therefore, it is difficult to control the sulfur content of the resultant compound. Thus, there remains a problem in that organic sulfur compounds having uniform structures can not be obtained.
Further, the produced compounds usually contain a large amount of polysulfide compounds having low or high molecular weights, and such a tendency is observed that, as the value of p in the formula: (CSw)p increases, the ratio of the above conjugated structure decreases and the ratio of the polysulfide compound increases. Particularly in case of a battery using an electrolytic solution (liquid electrolyte), the polysulfide compound or the polysulfide segment in the molecule of the above organic sulfur compound is decomposed in the course of charge and discharge and easily dissolved in the electrolytic solution, which results in the significant factors of lowering the stability of the compound and the stability of the battery comprising such a compound. As a result, the self-discharge of the compound itself becomes relatively large, and further, a metal sulfide which inhibits the reversibility of charge and discharge is formed, so that the cyclic life of the battery becomes shorter.
The first object of the present invention is to provide a polycarbon sulfide which has high reversibility and high capacity as an active material for electrodes and also has excellent stability.
The second object of the present invention is to provide a process for preparing the above specified excellent polycarbon sulfide.
The third object of the present invention is to provide a high-capacity nonaqueous electrolytic battery which is excellent in cyclic charge and discharge performance and reliability, using such an excellent polycarbon sulfide as an active material.
The present inventors have intensively researched in order to achieve the above objects, and found out a process for preparing a novel organic sulfur compound (polycarbon sulfide) from an organic sulfur compound comprising carbon and sulfur as constitutive elements and polysulfide segments, and the resultant compound substantially consists of two kinds of elements, i.e., carbon and sulfur and has a higher sulfur content and a higher uniformity in the molecular structure than the conventional organic sulfur compounds. The present invention is accomplished based on such a finding.
According to the first aspect of the present invention, there is provided a polycarbon sulfide comprising carbon and sulfur as constitutive elements and contains at least 67 wt. % of sulfur and at least 95 wt. % of carbon and sulfur in total, characterized in that a main peak appears at or around 1444 cmxe2x88x921 of a Raman shift in a Raman spectrum thereof, and that a peak within the range of 400 cmxe2x88x921 to 525 cmxe2x88x921 appears substantially only at or around 490 cmxe2x88x921.
According to the second aspect of the present invention, there is provided a polycarbon sulfide comprising carbon and sulfur as constitutive elements and contains at least 67 wt. % of sulfur and at least 95 wt. % of carbon and sulfur in total, characterized in that, in the X-ray diffraction with the CuKxcex1 ray, the diffraction pattern in the range of diffraction angle (2 xcex8) of 20xc2x0 to 30xc2x0 is shown substantially only by a broad diffraction peak having a peak at or around 25xc2x0.
According to the third aspect of the present invention, there is provided a polycarbon sulfide comprising carbon and sulfur as constitutive elements and contains at least 67 wt. % of sulfur and at least 95 wt. % of carbon and sulfur in total, characterized in that a decrease in weight, measured by the thermogravimetric analysis, is 5% or less when the compound is heated at a heating rate of 10xc2x0/min. from room temperature to 300xc2x0 C.
A polycarbon sulfide which comprises carbon and sulfur as constitutive elements and contains at least 67 wt. % of sulfur and at least 95 wt. % of carbon and sulfur in total, and which also has the above specified physical properties shows high capacity, high reversibility and a small decrease in capacity in association with cyclic charge and discharge when used as an active material for the electrode of a nonaqueous electrolytic battery. Therefore, the polycarbon sulfide of the present invention can provide a primary battery or a secondary battery with high reliability.
For example, the above polycarbon sulfide may be synthesized from an organic sulfur compound which comprises carbon and sulfur as the constitutive element and has a polysulfide segment represented by the formula: xe2x80x94Smxe2x80x94 (mxe2x89xa73) by eliminating a part of sulfur atoms composing the polysulfide segment, and converting the polysulfide segment into a disulfide linkage.
Further, the above polycarbon sulfide compound can be used as not only an active material for a nonaqueous electrolytic battery as exemplified above but also, for example, other electrochemical devices such as a capacitor, data memory devices, display devices and electronic materials.